Predefined Constants

Sort strings with different accents from the back of the string. This
attribute is automatically set to
On
for the French locales and a few others. Users normally would not need
to explicitly set this attribute. There is a string comparison
performance cost when it is set On,
but sort key length is unaffected. Possible values are:

The Alternate attribute is used to control the handling of the so called
variable characters in the UCA: whitespace, punctuation and symbols. If
Alternate is set to NonIgnorable
(N), then differences among these characters are of the same importance
as differences among letters. If Alternate is set to
Shifted
(S), then these characters are of only minor importance. The
Shifted value is often used in combination with
Strength
set to Quaternary. In such a case, whitespace, punctuation, and symbols
are considered when comparing strings, but only if all other aspects of
the strings (base letters, accents, and case) are identical. If
Alternate is not set to Shifted, then there is no difference between a
Strength of 3 and a Strength of 4. For more information and examples,
see Variable_Weighting in the
» UCA.
The reason the Alternate values are not simply
On and Off
is that additional Alternate values may be added in the future. The UCA
option Blanked is expressed with Strength set to 3, and Alternate set to
Shifted. The default for most locales is NonIgnorable. If Shifted is
selected, it may be slower if there are many strings that are the same
except for punctuation; sort key length will not be affected unless the
strength level is also increased.

The Case_First attribute is used to control whether uppercase letters
come before lowercase letters or vice versa, in the absence of other
differences in the strings. The possible values are
Uppercase_First
(U) and Lowercase_First
(L), plus the standard Default
and Off.
There is almost no difference between the Off and Lowercase_First
options in terms of results, so typically users will not use
Lowercase_First: only Off or Uppercase_First. (People interested in the
detailed differences between X and L should consult the Collation
Customization). Specifying either L or U won't affect string comparison
performance, but will affect the sort key length.

The Case_Level attribute is used when ignoring accents but not case. In
such a situation, set Strength to be Primary,
and Case_Level to be On.
In most locales, this setting is Off by default. There is a small
string comparison performance and sort key impact if this attribute is
set to be On.

The Normalization setting determines whether text is thoroughly
normalized or not in comparison. Even if the setting is off (which is
the default for many locales), text as represented in common usage will
compare correctly (for details, see UTN #5). Only if the accent marks
are in noncanonical order will there be a problem. If the setting is
On,
then the best results are guaranteed for all possible text input.
There is a medium string comparison performance cost if this attribute
is On,
depending on the frequency of sequences that require normalization.
There is no significant effect on sort key length. If the input text is
known to be in NFD or NFKD normalization forms, there is no need to
enable this Normalization option.

The ICU Collation Service supports many levels of comparison (named
"Levels", but also known as "Strengths"). Having these categories
enables ICU to sort strings precisely according to local conventions.
However, by allowing the levels to be selectively employed, searching
for a string in text can be performed with various matching conditions.
For more detailed information, see
collator_set_strength() chapter.

Compatibility with JIS x 4061 requires the introduction of an additional
level to distinguish Hiragana and Katakana characters. If compatibility
with that standard is required, then this attribute should be set
On,
and the strength set to Quaternary. This will affect sort key length
and string comparison string comparison performance.